1. Field of the Invention
The present invention relates generally to a heat exchanger for exchanging heat between two liquids, and more particularly to a heat exchanger that is adapted for cooling lubricating oil in an internal combustion engine of the type wherein a heat exchange element is made up of a plurality of stacked plate type heat exchange units.
2. Description of the Prior Art
In connection with heat exhangers or oil coolers adapted for cooling lubricating oil in an internal combustion engine, it is well known that a heat exchange element is made up of a plurality of stacked plate type heat exchange units. In addition, there has been proposed for such heat exhangers, to provide a bypass means in connection with the heat exchange element in order to allow lubricating oil to be discharged without being cooled when the oil temperature is considerably low, which is disclosed in U.S. Pat. No. 4,360,055 entitled "Heat Exchanger". The bypass means includes an oil pressure responsive leaf spring valve which controls the bypass flow of the lubricating oil through a bypass opening in response to oil pressure depending upon oil viscosity, so that the bypass opening is opened when the oil temperature is considerably low.
However, the following difficulties have been encountered in the heat exchanger provided with the above-stated bypass means: Fluttering of the leaf spring valve occurs under the action of intermittent impulsive pressure of the lubricating oil supplied to the heat exchanger element, thereby creating valve fluttering noise. Flow resistance of the lubricating oil passing through the bypass opening is increased at an initial period of valve opening in which the leaf spring valve has still partially covered the bypass opening. Furthermore, there is a possibility of malfunction of the valve in which the leaf spring valve is unavoidably opened in response to a raised flow resistance, for example, created due to clogging in an oil flow passage upstream of the valve, in spite of the fact that the viscosity of the oil is not high.